Being parasitic, one is a success by accident (5,5)
I believe the answer is:
liver fluke
'accident' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'being parasitic one is a success' is the wordplay.
I cannot really see how this works, but
'being' could be 'liver' (a being is something which lives) and 'liver' is located in the answer.
'success' could be 'fluke' (I have seen 'Accidental success' mean 'fluke' so perhaps 'success' could also mean 'fluke') and 'fluke' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'by' acts as a link.
Can you help me to learn more?
(Another definition for liver fluke that I've seen is " parasite".)